Fume hoods are enclosures used primarily in laboratories to carry out experiments and procedures that utilize or generate hazardous materials and contaminants. An opening is provided for access into the fume hood. The size of this opening may be changed by sliding a window which is mounted to move either vertically or horizontally. The window is typically moved by grasping a sash mounted along one side of the window. The window enables an operator to provide an opening just large enough to reach into the hood while visibly observing the experiment or procedure being conducted within the hood.
To remove hazardous gases and airborne materials, air is drawn through the opening and exhausted through an air duct typically located at the top of the fume hood. Moving air through the fume hood in this way minimizes exposure of the operator to the hazardous materials. It is important to regulate the velocity of air entering the fume hood (i.e., the face velocity). A face velocity that is too low will allow heavy gases and airborne particulate to spill out of the fume hood. A face velocity that is too high will create air turbulence that could eject the hazardous gases and particulate out through the fume hood opening.
Fume hood controllers of the type contemplated herein are used to control face velocity. Typical systems of this type are shown in U.S. Pat. No. 4,528,898, issued on July 16, 1985, to G. P. Sharp, et al., entitled "Fume Hood Controller," and U.S. Pat. No. 4,706,553, issued on Nov. 17, 1987, to G. P. Sharp, et al., entitled "Fume Hood Controller." Each of these patents is concerned with regulating the passage of air through the hood at a relatively constant face velocity by coordinating the position of the sash with the speed of a blower (i.e., fan) or the size of the opening of a damper (i.e., air valve) located in the exhaust duct. These control systems include a transducer for monitoring the position of the sash to provide a signal indicative of the area of the opening. In one system, the blower is maintained at a predetermined speed and the volume of air passing through the damper is controlled to provide a relatively constant face velocity through the hood. In an alternative system, a variable motor speed controller is responsive to the transducer to provide a blower speed which varies as a function of the sash opening.
The systems disclosed in these patents utilize open loop control which necessarily assumes that the damper and blower perform as they were designed and calibrated. However, the hood exhaust duct, damper, and blower are subject to corrosion and accumulation of exhausted materials over time which causes the performance characteristics of the damper and blower to change and which can unpredictably affect the flow rate of air through the exhaust duct and damper. Sensing the position of the damper does not account for these changes and therefore does not provide a reliable indication of air flow. Moreover, unless air flow is actually sensed, abnormal flow conditions, such as the blower stopping or an exhaust duct obstruction, may exist undetected.
Others have provided fume hood controllers utilizing closed loop control which provides greater accuracy than open loop systems and permits detection of abnormal flow conditions. Two examples of such systems are described in the Landis & Gyr Powers VAV Fume Hood Control System brochure and the Krueger System 3 Custom DDC VAV Laboratory Ventilation Equipment brochure.
In fume hood control assemblies, it is critical that the necessary change in air flow occur within four seconds after the sash position has changed. Otherwise, hazardous gases and airborne particulate may escape through the fume hood opening. In closed loop control of fume hood face velocity, there is an inverse relationship between system response speed and system stability. Therefore, faster response can only be obtained at the expense of stability. Configuring a closed loop system to control air flow within the critical four seconds results in an undesirably unstable response that, at a minimum, lasts for several minutes. Thus, there is a need for a fume hood controller that provides fast, stable response to changes in sash position while maintaining accurate control of the face velocity of air entering the fume hood opening.